Do you actually delete old pages? (and then rename and reupload, or sth)
Do you have a lot of links, bit of unrelated, in your pages?
Maybe are you having duplicated content, paragraphs, pages? Too many ads in many pages?

How is your domain? In which kind of hosting it is? It sometimes depends on if hosted in certain kind of hosting, or if the url is of certain kind, if for the domain you are using redirections, cloaking...

A lot of factors can make a site go down, I'd check most of what is usually said to lower it or possibly harm it.
Just some brainstoprming to help.